PMBX No. 12

Descriptive Leaflet

SEPT 1978
B 36/78

PMBX No. 12

This Circular describes the new Premiere Call Connect System, which is now nationally available, and details Marketing Policy, Tariff, Sales and Engineering procedures.

The Premiere Call Connect System is a prestige medium sized switchboard, designed to meet the continuing needs of customers who prefer to have the personal service provided by a switchboard operator. It is styled to suit contemporary office decor. The modern cordless desk-top operator's console is push button key operated and the system equipment is mounted on slide-in modules that are contained in a compact free standing cabinet. Direct Outward Dialling (DOD) from extensions to the public exchange is available as an option and automatic clearing of calls helps to reduce the operator's work load.

This is the forerunner of a new series of modular Call Connect Systems providing greater flexibility for equipment provisioning.

The Premiere has a maximum capacity of 10 exchange lines and 48 extensions; its modular design, based on slide-in units, allows initial requirements and subsequent growth to be supplied in modules of 1 exchange line or 4 extensions.

Connect circuits for the connection of extension to extension calls are also provided on slide-in "connector modules". The Premiere is equipped with a minimum of 2 such connector modules but has capacity for a maximum of 4; if the traffic justifies a third or fourth connector module it may be supplied and the customer will be charged accordingly.

Inter-PBX private circuits and inter-PBX extensions, up to a maximum of 4 may be provided but their use reduces the exchange line capacity by 1 for each circuit provided. In addition, each inter-PBX extension must be allocated an extension push button key on the switchboard console and this reduces the extension capacity. The signalling systems available are Manual Balanced Battery (M/BB) and Signalling System SCDC (SSDC10).

The switchboard console is a cordless light grey desk-top unit with wooden ends. It measures 584mm wide (23 in), 255mm deep (10 in), 178mm high (7 in) and weighs 10.5kg (23 lb). All of the exchange lines, extensions and inter-PBX circuits and connect circuits appear on push-button keys and there is a 12 push-button keypad for setting up calls over the public network. Calls are indicated by lamps flashing inside the appropriate push-button keys. A light-weight Trimphone handset (No. 10A), is provided for the operator; a light weight headset (No. 1 or No. 2) may also be provided at standard additional charge.

The Premiere features automatic clearing of connections and this, together with the optional direct outward dialling facility helps to reduce the operator load and so make the installation particularly suitable for operator/reception locations.

The associated equipment and the termination of the distribution wiring are contained within a matching free-standing cabinet, styled to be compatible with modern office furniture. It measures 900 mm wide (35 in), 600 m deep (23 in), 1300 mm high (51 in), weight 200.0 kg (450 lb) fully equipped, 90.0 kg (200 lb) unequipped and is designed to be installed in the same room as the console since the equipment is quiet in operation. The equipment cabinet is connected to the operator's console by 2 cords, which are provided to a standard length of 3 metres. Longer cables may be provided to meet special needs if necessary at additional assessed charges. It may stand against a wall as maintenance access is from the front and top covers but customers should be requested to keep the top of the cabinet clear to permit ready access by PO staff. The system is equipped with a combined power unit and ringing converter (No. 148A) which is housed in the bottom of the equipment cabinet. It is operated from a 200/250v, 50 Hz supply via a 3 pin switched socket outlet of at least 13 Amps rating which must be provided by the customer. Power consumption does not normally exceed 3 Amps.

The equipment cabinet can be equipped with up to the following maximum quantities of plug-in modules (Apparatus Slide-in Units ASUs):-

12 x Extension Line Circuit Units (4 extensions per unit)
10 x Exchange Line or Inter-PBX Circuit Units (maximum of 4 inter-PBX Circuit units)
4 x Connector Units (minimum of 2 must be provided)
1 x Position and Miscellaneous Circuit Unit

Other optional ASUs can be provided up to the following quantities:-

1 x Predetermined Night Service Unit
1 x Direct Outward Dialling (DOD) Common Control Unit
4 x Direct Outward Dialling (DOD) Auxiliary Extension Line Units each providing 12 circuits

From the issue of this Circular the "PREMIERE CALL CONNECT SYSTEM is nationally available and should be offered to:-

  1. New customers with a potential requirement exceeding 4 exchange line and 18 extensions, but not more than 10 exchange lines and 48 extensions, whose needs are best met by the provision of a manual system.
  2. Existing customers who, upon a change of system, would fall into the category as outlined in 1. There is good potential for converting existing PMBX 10+30/10+50 customers to the Premiere Call Connect System.
  3. Orders for supply of the Premiere Call Connect System may now be negotiated. Engineering information to enable stores to be ordered will be avail- able from the end of January next year but preliminary planning for accommodation and distribution wiring may proceed with the view to bringing first installations into service as soon as practical after receipt in areas of detailed Installation Instructions.

When discussing the Premiere Call Connect System with customers the following points should be borne in mind:-

  1. The product name of "Premiere Call Connect System", should be used in full. It should not be reduced to initial letters.
  2. The Premiere is equipped with 2 connect modules for the connecting of extension to extension calls; these should be adequate to meet the traffic needs of most customers having up to 24 extensions. A fully equipped system with 48 extensions is likely to justify 4 connect modules unless, for example, the customer has a PAX catering for extension to extension traffic. If the customer requires a traffic record the Area Traffic Division should be consulted.

    The customer should be informed that the third and fourth connect modules incur additional charges.
  3. As Direct Outward Dialling (DOD) is a major feature of the Premiere Call Connect System, the advantages of increased efficiency and better utilisation of time by extension users and the console operator should be fully impressed upon customers. At the same time, the absence of operator control of extension dialled calls should be fully understood by them. DOD may be supplied at a fixed additional charge irrespective of the number of extensions to be so equipped. When a customer wants DOD at his installation he must decide from which of his extensions it is to be accessed.
  4. Night Service Arrangements: Consideration must be given at the outset to the arrangements required for night service and mains failure.
  5. Allocation of inter-PBX Numbers: Inter-PBX private circuits reduce the exchange line capacity by one for each circuit provided and use either exchange line positions 10, 9, 8 or 7. In addition to the exchange line position each inter-PBX extension requires an extension push-button key and extension numbers 48, 47, 46 and 45 must be used for this purpose.
  6. Descriptive Leaflet DLC 340 is available.

Introduced in 1978.

MAR 1980

PMBX No. 12
(As this Instruction has been completely REVISED, individual paragraphs
have not been "starred".)


  1. INDEX
  7. POWER UNIT 148A

Premiere is a cordless system comprising a switchboard and an equipment cubicle. The design employs sealed dry reed switching logic, with an optimised arrangement of solid state electronics and electro-mechanical relays for control functions. It has a maximum capacity of 10 Exchange Lines and 48 Extensions, but up to 4 Inter PBX Private Circuits or Extensions-can be provided by utilising spare Exchange line positions.

Additional features provided by the PMBX No. 12 are:-

  • Direct Outward Dialling, which can be provided at any designated extension.
  • Automatic Call Clearing and ringing.

The Switchboard PMBX No. 12 is a cordless desk unit employing illuminated push button keys, a Handset No. 10A-2, a Push Button Keysender, and is supplied fully equipped.

The associated equipment cubicle, Equipment PMBX No. 12 which includes a cross connection field can be equipped up to the following maximum quantities of Apparatus Plug-in Units (PIUs):-

12 - Extension Line Circuits (4 Extensions per unit)
10 - Exchange Line, or Inter PBX Units
4 - Local Connector Units
1 - Position and Misc Circuit

Other optional PIUs can be provided up to the following quantities:-
1 - Predetermined Night Service Unit
1 - Direct Outward Dialling (DOD). Common Control Unit
4 - Direct Outward Dialling (DOD). Auxiliary Extension Line Units each providing 12 circuits.

Access to the equipment cubicle is by means of the top cover (see para 6.2.1). Customers should be requested to keep the top of the cubicle clear to permit easy access by PO staff.

A Circuit Diagram Folder (CDF 15) is provided at each installation. The contents of the CDF are listed in Table 1. Also available to assist both installation and maintenance engineers is an Equipment Handbook. (Customers Apparatus Guide Notes. The Premiere Call Connect System - PMBX 12).

Other related instructions are:-

C3 E5040 - Description and Facilities
E16 A0013 - Addresses of Area Repair Centre (ARC)
El D1416 - Piece Parts

Storage space is available within the equipment cubicle to accommodate the local records, diagrams, Equipment Handbook and small stores required for normal maintenance e.g. Lamps, Fuses and Jumper Wire.



Trunking Diagram SA 10175
Wiring Tables SAW 101750
Equipment Cubicle Wiring SAJ 101750
Cross Connexion Diagram SAX 101750
Local Connector SA/SAW 101760
Exchange Line Connector SA/SAW 101770
Position and Misc Circuit SA/SAW 101780
Extension Line Circuit SA/SAW 101790
DOD Access Circuit SA/SAW 101810
DOD Common Control Circuit SA/SAW 101820
SCDC Signalling SA/SAW 101830
Manual Balanced Battery -
Signalling (M/BB) SA/SAW 101840
Operators Cordless Switchboard Cct SA/SAW 101850
Keysender No. 11A SA/SAW 101880
Common Services SA/SAW 101900
SS DC5 Signalling SA/SAW 101910
Limiter No. l/SA 101920 SA/SAW 101920
Dial Unit SA 10204 SA/SAW 102040
Power Unit No. 148A SA/SAW 101870
Installation Specification S 1368
Operating Instruction Booklet A4077

The design achieves high system reliability without routine attention allowing corrective maintenance procedures to be applied. Rapid restoration of service is aided by its modular assembly in which circuits are accommodated in easily replaced plug in units (PIUs). Para 12 gives guidance on known operational difficulties. In the event of a mains power failure, all existing calls are lost and the Exchange lines are automatically switched to the Extensions designated under Night Service 2 arrangement. When power is restored, existing calls are maintained.

5.1 Equipment PMBX No. 12
Maintenance within the equipment cubicle will he by replacement or transposition of the suspect PIU to indentify the faulty unit, which should be replaced by a maintenance replacement unit. An Extractor No. 37A is required to remove PIUs.

Faulty units should be inspected on site and repaired if appropriate e.g. in PIUs look for pinched wires under springs, broken wires or components, displaced 3000 Type Relay armatures and check that the Type 23 Relay armatures are not stuck by inserting a short length of wire (18 swg), a straightened paper clip will do, into the small hole at the back of the relay to manually operate it.

5.2 Switchboard PMBX No. 12
Faulty lamps, key senders, buzzers, Bracket Telephone No. 19A, Handset No. 10A-2, Telephone Unit, broken or lost key tops can be replaced on site. However broken or faulty push button keys should not be changed on site, but the complete switchboard replaced with a maintenance replacement unit. An Extractor No. 4 is required to remove key tops/lamp caps, when changing lamps.

5.2.1 Standby Dial
A standby dial is provided and should be left with the operator, or other responsible representative of the customer, for use when required.

The Standby Dial Unit plugs into the jack situated on the base of the switchboard adjacent to the equipment cubicle cord jacks. Plugging the unit into this jack inhibits the self-contained keysender, the switchboard can then be used until a replacement keysender is fitted.

The Standby Dial Unit is coded as a Dial Unit SA 10204 and comprises:-

  • Dial Auto 21FA
  • Mounting Dial Auto No. 22A complete Cord Instrument 5/19L, 22"
  • Plug 420 Grey with Cord 1 /77AB, 54"

6.1 Switchboard PMBX No. 12

6.1.1 To change the lamps, remove the keytops/lamp caps with the Extractor No. 4; withdraw the Lamp No. 2/6 with the Lamp Extractor No. 5. Replace in reverse order.

6.1.2 To gain access to the Switchboard PMBX No. 12 it should first be stood on its side and the four fixings screws situated at the corners of the base removed to release the upper section. The two sections should then be lowered onto the base together; the Keyboard or upper section can now be raised independently of the base, giving access to the Keysender No. 11A, Buzzer No. 33A and the Handset rest, Bracket Telephone No. 19A.

6.1.3 To change a Faulty Keysender No. 11A unplug the connecting cord from the socket, release the two fixings holding the keysender and remove the complete unit from the switchboard front panel. Fit the maintenance replacement, using the reverse order.

6.1.4 To change the Buzzer No. 33A remove the spade-ended wiring from the connecting screws, noting their relative positions, and unclip the faulty buzzer. Fit the maintenance replacement using the reverse order.

6.1.5 To replace a broken handset rest (Bracket Telephone No. 19A) release the two fixings and remove; use the reverse order to fit the maintenance replacement.

6.2 Equipment PMBX No. 12
6.2.1 To gain access to the Equipment cubicle twist the six screw fasteners on the top panel half a turn in an anti-clockwise direction. The top panel can then be lifted from the front and removed. This panel of the cubicle has a diagram on the underside showing the tag block disposition and identification. With the top panel removed, the complete cross connection field is exposed for cabling and jumpering.

All external cabling to the equipment cubicle should be fed into either of the side access points, but not both. This will permit access to be gained to the back plane of the equipment rack, by turning the whole cubicle through 90 degrees , should this be necessary.

6.2.2 Access to the PIUs and the combined power unit and ringing converter (Power Unit No. 148A) is gained after removing the top panel (see para 6.2.1) and then removing the front panel by releasing both locks and gently easing the front panel forward from the top and lifting clear. Should the rubber anti-rattle strip at the bottom of the cubicle become dislodged, this should be replaced immediately, to prevent loss or damage, whilst working on the equipment.

6.3 Removal and Replacement of PIUs
Use an Extractor No. 37A to remove Plug-in Units (PIUs) from the equipment rack. A firm straight pull is required to move the PIU out approximately 10 cm (4 in). Replace the extractor into the storage place, then, using both hands, the PIU should be completely removed taking care not to touch the gold-plated edge contacts. When replacing the PIU, take care to ensure that the PIU runner engages correctly in the guides, then gently slide the PIU in until the initial resistance of the plug engaging with the socket is felt. This occurs when the PIU front panel is approximately 1 cm (0.5 in) from its final position. A firm push should then be given to engage the plug and socket.

Cubicle:- 6.3.1
Checks to be made before inserting PIUs into the Equipment:-

  • Verify that the correct PIU has been selected for insertion into the particular position.
  • Check no wires are trapped in the PIU guide assemblies.
  • Inspect the components and wiring to ensure that they have not been accidentally displaced or damaged in transit.
  • Check facility, or test, links are correctly located.

Damage or operational difficulties can result if any of the above checks are not carried out.

The combined power unit and ringing converter is a free standing unit situated at the bottom of the equipment cubicle. Apart from replacement of fuses and ensuring that the output plug is correctly inserted into the socket, no other maintenance is required.

A Kit No. 336A containing:-

3 Fuses 72/0.25
3 Fuses 72/0.5
4 Fuses 72/3.15

is provided at each installation for maintenance purposes.

Replace faulty units with a maintenance spare.

NOTE: Before proceeding with any work on the Power Unit No. 148A switch off the Mains Supply and isolate the unit by removing the mains plug from the socket.

The discrete component technology employed in the construction of PIUs facilitates a measure of local repair. When, in the opinion of the Technical Officer concerned with maintenance at the installation, an effective repair can be carried out using existing off-site facilities this should be done. This includes faulty PIUs identified by the maintenance Technician. Otherwise faulty PIUs should be sent for repair at an Area Repair Centre. (ARC).

Facilities have been made available for repair of PIUs and Switchboards PMBX No. 12 (Cordless Desk Units) in ARCs. TI E16 A0013 gives the addresses of ARCs.

All faulty equipment must be packed with care in the original packing of the replacement unit before it is returned for repair, also enclose a note of explanation of the difficulty encountered. Include your name, address and telephone number for the return of repaired item. A form A646 should also be completed and forwarded via your supervising officer to the Regional Service Group who in turn should process it appropriate manner.

The latter A646 procedure is required by the Quality Assurance Division at present due to the suspect quality of new equipment being supplied. The use of the form A646 for this purpose should be ceased at the end of June 1981 and the form then only used for its intended purpose i.e. Report of Unusual Difficulty or Defect.

Alterations to the following facilities may be carried out during the course of a normal maintenance visit:-

Night Service 1
Night Service 2
Direct Outward Dialling

However the A44 procedure must be used to account the charges for the changes and to ensure that the customers records are updated. All changes should be entered on form A863 and a copy left with a responsible representative of the customer.

NOTE: Night Service 2 designated extensions must be the same as those selected for the 'Mains Fail' options.

Normal lightning protection is provided in accordance with TI A2 E0504. The Position and Misc PIU has a surge limiter fitted across the -50 volt supply rail. In areas of known high risk or sites with a previous history of lightning damage a limiter l/SA 10192 should be fitted on all exchange line PIUs.

The limiter is connected to the violet wire where this is run adjacent to the upper frame of the PIU. The large holes are provided in the circuit board of the limiter and these are used with suitable self tapping screws to attach the limiter to the upper frame as near as is practicable to the PIU back panel. (Components should face towards the PWB.)
The violet wire should be cut approximately 5 cm (2 in) from the edge connector and both ends stripped of 2 mm (A ins) of insulation. Both ends of the violet should be soldered to the -50 V points on the limiter, the earth connection is made by the two self tapping screws securing the limiter to the PIU

At installations where a major failure has occurred the normal operating procedure will be to switch to the Night Service 2 option (Mains Fail) by operating the NS2 Key on the switchboard. Exceptionally however this will not be possible, if certain relays are being held operated by the fault condition: under these circumstances the mains supply should be switched off at the supply socket.

12.1 Changing Lamps Difficulty in changing switchboard lamps will be experienced unless the correct tools are used (see Para 6.1.1).

12.2 DOD
When Connected to TXK3 or Line Finder TXS Extensions using DOD can experience difficulty when the PMBX No. 12 is connected to a TXK3 or Line Finder TXS type public exchanges, although calls via the switchboard operator are satisfactory. The problem arises when a DOD extension calls the public exchange; while waiting for dial tone the line current value can be below that required to operate the line relay in the PMBX No. 12 exchange line PIU so the call releases, which causes the extension to inadvertently call the switchboard operator. This difficulty does not occur if dial tone, and the resulting increase in line current is returned to the PMBX No. 12 within normal times of the DOD extension originating the call.

To correct this difficulty, change the line relay (LS) from a 3000 Type Relay No. 17707 to a No. 6379.

12.3 Effects of Extension PIU Faults
A fault in an extension PIU can cause difficulty in connecting exchange lines or local connectors to any extension telephones. First the exchange line and then the local connector PIUs should be moved to different positions to determine if the fault moves accordingly. If however, the fault does not move but remains associated with the original circuit then a stuck reed relay in an extension PIU associated with the particular circuit is the likely cause. As the exchange line and local connectors have their cross points in all extension PIUs the faulty PIU has to be identified and changed.

12.4 Identifying a Faulty Extension PIU:-

  • Set up a call and prove the fault condition. Clear call and press "Release Answer" Key.
  • Repeat above test with all DOD PIUs removed.
  • Withdraw each extension PIU in turn repeating the test in (i) until the faulty PIU is identified.

As all extension PIUs are identical, the remaining PIU should be positioned to give the customer the best possible service until the faulty PIU is replaced by a maintenance replacement unit.

12.5 Power Unit Failures
A complete system failure will occur should the Power Unit 148A fail. A failure of the negative 50 volt can be identified by the "Green" lamp on the switchboard being out. See Para 7. To test other outputs refer to diagram SA/SAW 101870.

12.6 Effects of Position and Miscellaneous PIU Failure
Failure of the Position and Miscellaneous PIU can also cause a complete system failure. Other fault conditions that can result from failure of this PIU are as follows:-

  • No incoming signals.
  • Flashing rate of incoming, and hold the signals incorrect. Possibly the signals may be permanent or no signals depending on the mode of failure.
  • Interrupt tone too loud or not working.
  • Failure to connect circuits.
  • Buzzer circuit faults. (Permanent/Faint buzzer faults are usually due to a transistor fault in other PIUs. Remove in turn all PIUs until the faulty PIU is identified.

12.7 False Extension Call Signal on Switchboard
To prevent misoperation of the extension recall button while the handset is on hook, from calling the switchboard operator; it is advisable to connect the 'A' wire to Telephone T9 and the 'B' wire to Telephone T18. This is the reverse to that shown in the telephone diagrams.

12.8 Keysender Failure
The Keysender can fail in such a way as to prevent the operator from answering or originating exchange line calls. To overcome this difficulty a standby dial is provided and should be plugged into the Jack situated on the switchboard lower panel adjacent to the equipment cubicle cord jacks, this inhibits the self-contained Keysender.

In the absence of a Standby Dial, a Plug 420 (no strapping or connections are necessary within the plug) can be inserted into the Jack to inhibit Keysender malfunction. The Operator can then answer incoming exchange calls, and give extension users exchange lines to originate their own outgoing calls until such times as the repair is effected.

12.9 No Busy Test
On restoration from night to day service or on power restoration after mains fail, any exchange line calls in progress are held connected. If the operator attempts to extend a further call to the extension so engaged no busy test is given, neither is the existing call interrupted. However, following cleardown of the call(s) in progress all functions become normal.

12.10 Access to Out of Area Lines
Out of area exchange lines may be unintentionally siezed by DOD extensions. To overcome this difficulty complete the following modification:-

  • Using a two wire jumper plus its connector join both Red and Blue wires together.
  • Push connector on to tags 6-206 of exchange line jack concerned.
  • Put a paper sleeve on contact spring MF6 (3000 type Relay) in exchange line PIU.
  • Label front panel of PIU for identification, and enter modification into local records.

MAR 1980

Piece Parts



This instruction details the piece parts that may be changed on site, and it is essential that a supply of maintenance replacement items be available when a PMBX No. 12 is installed. For up to the first 10 systems one set of spares should be held. Over 10 systems a 10% holding should be maintained for Position and Miscellaneous and DOD Common Control Plug in Units (PIUs). The holding of all other items should be adjusted according to trend as the number of systems increases.

Items that should not be included in the 10% spares holdings are:-
Switchboard, Equipment Cubicle and Power Unit 148A.

These items are both large and costly, and should be held on a Regional basis. Each Regional Service Group should designate where these items and to be kept and Area supervising officers should contact their own Regional officer should any of these items be required.


Bracket Telephone 19A 13 5794
Buzzer 33A 37 0093
Dial Unit 5A 102014 (Standby Dial) 37 3243
Handset 10A-2 complete with Plug and Cord 4/117AX 37 3245
Plug 1420 grey with cord inst 4/117AX Green 200 mm 43 4645
Headset No. 1 37 0886
Plug 420 Grey with cord 4/77AB 43 2776
Keysender No. 11A 41 0483
Key Button 3B-1 (Key Tops Blank) * 43 4943
Lamps No. 2/6v Grey 43 1856
Telephone Unit (PWB) Requires Modification 43 4493
Switchboard PMBX No. 12 (Complete) 37 3198

* There are 67 differently designated Key Tops used, but for economy only a blank is listed.


Local Connector circuit 37 3188
Exchange Line circuit 37 3189 
Position and Miscellaneous circuit 37 3190
Extension Line circuit 37 3191
Pre-determined Night Service circuit 37 3192
DOD Access circuit 37 3193 
DOD Common Control circuit 37 3194
Inter PBX SCDC circuit 37 3195
Inter PBX M/BB circuit 37 3196
Equipment Cubicle PMBX No. 12 (complete with Kit 326A) 37 3197
Power Unit 148A 31 4118
Fuse No. 72/0.25 31 4215
Fuse No. 72/0.5 31 4216
Fuse No. 72/3.15 31 4217 
Cubicle Fuses See Table 3
Cords Connecting No. 100/2A (Cubicle to Switchboard) See Table 3


Cord Connecting No. 100/2A (Cubicle to Switchboard) 51 2210
Cords Connecting No. 100/2A - 6, 8 or 10 meter lengths Note

Note - These should be ordered direct from the manufacturers using the following description:-
Cable Assembly Tekdata Specification SD 4874 issue D
State length 6, 8 or 10 meters. PO description Cord Connecting 100/2A.

Direct to:-
The Sales Manager
Westport Lake
Canal Lane

Cubicle Fuses
Fuse 0.55A Green - 344 CPR 100014 AAD
Fuse 1.7A Grey - 34 CPR 10004 AAC


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Last revised: January 18, 2022